Field of the Invention
The present invention is directed to solid electrolytes, methods of making the solid electrolytes, and methods of using the solid electrolytes in batteries and other electrochemical technologies.
Background
The development of high capacity, high power, and low cost electrochemical batteries will help to catalyze the coming energy revolution. For example, inconsistent production of energy could be averaged over several days or weeks with large, inexpensive batteries. And, vehicles could run on electric motors powered, by energy dense batteries, ultimately drawing their power from the electric grid.
Lithium-ion batteries are some of the best performing and most prevalent batteries today. Unfortunately, they bring with them a host of problems, most notably a limited supply and the high cost of lithium. See “The Trouble with Lithium 2: Under the Microscope,” Meridian International Research 2008. Current lithium-ion batteries also entail short cycle-life and dangerous overheating scenarios. See J. J. Ciesla, J. Power Sources 18:101-107 (1986). Moreover, conventional lithium-ion batteries suffer drawbacks due to their organic liquid electrolyte, including dissolution of electrodes into the electrolyte and development of a “solid-electrolyte interface” (SEI), which decreases round trip efficiency and greatly shortens cycle life. See Arora, P., et al., J. Electrochemical Soc. 145:3647-3667 (1998); Smart, M. C., et al., J. Electrochemical Soc. 146:3963-3969 (1999); Blyr, A., et al., J. Electrochemical Soc. 145:194-209 (1998); and Y. Shin and A. Manthiram, Chem. of Materials 15:2954-2961 (2003). Further, the breakdown voltage of liquid elecrolytes is only 4V, and liquid electrolytes have been shown to out-gas and explode, which limits the operating voltage and temperature of the battery. See M. Na, Solid State Ionics 124: 201-211 (1999); Y. Shin and A. Manthiram, Chem. of Materials 15:2954-2961 (2003); K. Xu, Chem. Rev. 104:4303-4417 (2004); J. J. Ciesla, J. Power Sources 18:101-107 (1986); Wang, Q. S., et al., Electrochemical and Solid State Letters 8:A467-A470 (2005), Hyung, Y. E., et al., J. Power Sources 119:383-387 (2003); and Xiang, H. F., J. Power Sources 173:562-564 (2007).
All-solid-state sodium-ion batteries promise a cheap, safe alternative to current battery chemistries. Solid state ceramic electrolytes show no electrode dissolution or SEI formation, have been shown to be stable beyond 5V (see Hayashi, A., et al., Nature Commun. 3:856 (2012) and Bates, J. B., et al., J. Electrochemical Soc. 142:L149-L151 (1995)), and are safe to use at very high temperatures due to the intrinsic stability of ceramics. However, the room temperature conductivities of ceramic sodium electrolytes are usually several orders of magnitude lower than their organic counterparts. See J. W. Fergus, Solid State Ionics 227:102-112 (2012), E. J. Plichta and W. K. Behl, J. Power Sources 88:192-196 (2000), and N. D. Cvjeticanin and S. Mentus, Phys. Chem. Chem. Phys. 1:5157-5161 (1999).
If solid-state sodium-ion batteries are to be competitive, they must have high performance at room temperature, thus high conductivity solid electrolytes. Superionic NASICON (Na+ Superionic Conductor), Na3Zr2Si2PO12, is one of the most promising and widely studied solid electrolytes. However, the conventional formulation of NASICON provides insufficient performance at room temperature, requiring the use of the higher temperature rhombohedral phase.
Several studies have been published investigating trends in doping effects. See Miyajima, Y., et al., Solid State Ionics 124:201-211 (1999); M. Na, Solid State Ionics 124:201-211 (1999); Saito, Y., et al., Solid State Ionics 58:327-331 (1992); Takahashi, T., et al., Solid State Ionics 1:163-175 (1980); A. Feltz and S. Barth, Solid State Ionics 9:817-821 (1983); and Miyajima, Y., et al., Solid State Ionics 84:61-64 (1996). It has been demonstrated that ionic conductivity increases with transition metal radius doped at the octahedral zirconium site See Saito, Y., et al., Solid State Ionics 58:327-331 (1992) and Miyajima, Y., et al., Solid State Ionics 84:61-64 (1996). However, the evidence for this trend has been drawn from doping the silicon-free and much less conductive NaZrP3O12 (NZP) compositional end member. Unlike NaZrP3O12 which is stable in the rhombohedral structure at room temperature, Na3Zr2Si2PO12 shows a transition to a low temperature monoclinic phase around 175° C. See Alpen, U. V. et al., Materials Research Bulletin 14:1317-1322 (1979); Feist, T., et al., Thermochemica Acta 106:57-61 (106); and Bukun, N. G., Ionics 2:63-68 (1996).
Following this trend to the extreme, Miyajima showed that the largest ion soluble in NZP is dyspropium, after which maximum solid solubility drops to near dilute doping levels. See Miyajima, Y., et al., Solid State Ionics 84:61-64 (1996).
No such trend has been discovered in superionic NASICON. Solid solubility in NASICON may not be as tolerant to large radii, as suggested by Takahashi's observation of low yttrium solubility in NASICON despite the largest dysprosium's solubility in NZP. See Takahashi, T., et al., Solid State Ionics 1:163-167 (1980).
The present invention provides several NASICON type materials where the metal ion is sodium that have been doped with octahedral site dopants having different ionic radii. Surprisingly, it has been found that a smaller ionic radius trivalent dopant can increase the conductivity of the solid electrolyte.